The present invention relates to an epoxy resin-based, anticorrosive primer composition suitable for use in the production of anticorrosive steel sheets, particularly those for use in automobiles. The anticorrosive primer composition of the present invention is capable of forming a coating film having improved press formability, corrosion resistance, and electrodeposition coatability.
Anticorrosive steel sheets which have been used in the manufacture of automobile bodies include (1) zinc-plated steel sheets having a plated coating of zinc or a zinc-based alloy formed by electroplating or hot dipping on a substrate steel sheet, (2) chemically-treated, zinc-plated steel sheets having a chemical conversion coating such as a chromate coating on a zinc-plated steel sheet, and (3 ) organic-coated steel sheets having a thin organic coating formed by application of an anticorrosive primer on a chemically-treated, zinc-plated steel sheet (hereinafter referred to as organic-coated, anticorrosive steel sheets). The anticorrosive ability or corrosion resistance of these steel sheets (1), (2), and (3) increases in that order.
Particularly, organic-coated, anticorrosive steel sheets which have a three-layered protective film consisting of a base zinc-plated coating, a chromate coating, and an upper organic coating film, possess an outstandingly superior corrosion resistance among various surface-treated steel sheets due to the excellent anticorrosive effect of the zinc-based plated coating and the chromate conversion coating associated with the shielding effect of the organic coating film as a barrier to shield from the surrounding environment.
In recent years, as the level of corrosion resistance required for various steel products including automobiles becomes higher, the demand for organic-coated, anticorrosive steel sheets increases. In applications for manufacturing automobile bodies, it is desired that the protective film of such an organic-coated, anticorrosive steel sheet not only exhibit excellent corrosion resistance over a prolonged period of time but also have good electrodeposition coatability and press formability.
It is known that the corrosion resistance of an organic-coated, anticorrosive steel sheet can be further improved by incorporation of fine silica particles such as colloidal silica in the upper organic coating layer thereof. For this purpose, it is desired that silica particles be dispersed uniformly and stably in a coating composition (anticorrosive primer) used to form the organic coating, because otherwise the resulting silica-containing organic coating may have corrosion resistance and other properties which are uneven over the entire surface and a deteriorated surface appearance. Therefore, the stability of dispersed silica particles is important for a silica-containing anticorrosive primer.
Silica-containing coating compositions suitable for use as anticorrosive primers in the formation of organic coatings for anticorrosive steel sheets are disclosed, e.g., in Japanese Patent Application Kokai No. 2-134238 (1990) and U.S. Pat. No. 5,147,729.
The coating composition described in Japanese Patent Application Kokai No. 2-134238(1990) comprises 100 parts by weight of a resin composition, which is a reaction product of a urethane-modified epoxy resin having an epoxy equivalent weight in the range of 1000-5000 with an alkanolamine to add to the epoxy groups in the resin, and from 10 to 150 parts by weight of silica on a solids basis. The urethane-modified epoxy resin is prepared from a mixture of an epichlorohydrin-bisphenol A epoxy resin and a polyalkylene glycol diglycidyl ester by reacting it with bisphenol A followed by a diisocyanate compound.
U.S. Pat. No. 5,147,729 describes a coating composition comprising 100 parts by weight of a resin composition, which is a reaction product of a urethane-modified epoxy resin having an epoxy equivalent weight of 1000-5000 (prepared by reacting a bisphenol A epoxy resin with an isocyanate compound) and a dialkanolamine to add to the epoxy groups of the resin, and from 10 to 150 parts by weight of silica on a solids basis.
However, none of the silica-containing coating compositions, including the above-described ones, known in the prior art as useful in the formation of an organic coating for anticorrosive steel sheets provide satisfactory results with respect to all the required properties of stability of silica particles dispersed in the composition and the corrosion resistance, press formability, and electrodeposition coatability of the resulting organic coating formed from the composition.